1. Field of the Invention
The present invention relates to a steel for high-strength spring which is used as a spring part for vehicle, a method for producing the same, and a high-strength spring produced by the method.
2. Description of the Related Art
Under an environmental assessment to prevent global warming, it has been desired to attain lighter bodies in order to control the emission of carbon dioxide gas in the automobile industry. An approach for achieving the demand for lighter bodies includes enhancing the strength of a coil spring for supporting bodies. On the other hand, the coil spring for supporting bodies is exposed to a severe corrosive environment, and thus the durability is required.
In order to ensure the durability of the coil spring in the corrosive environment, the research and development for improving the corrosion resistance of materials by adding alloy elements such as Ni, Cr, Mo, and V, which are effective for corrosion resistance, have been carried out and various types of proposals have been made. As for the means for improving the corrosion resistance and delayed fracture resistance, for example, in the case of a high-strength steel for coil spring produced by adding expensive alloy elements, such as Ni, Cr, Mo, and V (hereinafter referred to as “existing high Cr steel”) described in Patent Literature 1, the additive amount of each alloy element is increased as the usage environment becomes severe. Consequently, the material cost is increased. A large additive amount of Cr causes not only high-cost but also instability of a raw material supply system. Physically, this leads to a disadvantage that the corrosion pit becomes an acute angle. That is, Cr acts positively on the general corrosion, while it acts negatively on the corrosion pit (pitting), i.e., a trade-off relationship. It is unclear where the optimal value of the Cr content as the additive amount exists.
Further, Ni has an effect of improving the corrosion resistance and an effect of increasing the amorphous composition of the rust and decreasing the aspect ratio of the corrosion pit. This causes not only an increase in cost but also instability of a raw material supply system because Ni-producing countries and areas are unevenly distributed.
From such a background, the study for eliminating the unstable factors has been further proceeded and various types of proposals have been made. For example, Patent Literature 2 describes a high-strength steel for coil spring which is produced to have a desired high strength with a low Cr content and a low Ni content. In Patent Literature 2, it is suggested that composition ratios of constituent elements are specified using three parameters Ceq 1, Ceq 2, and Ceq 3 in order to realize the steel for high-strength spring which contains a small amount of alloys.